1. Field of the Invention:
The present invention relates to an optical recording medium, and particularly to an optical recording medium using an organic dye material as a recording layer.
2. Description of the Related Art:
An optical recording medium using an optical organic dye material as a recording layer includes a transparent substrate made of a suitable material such as a plastic on which a recording layer having an organic dye as a main component, a reflective layer, a protective layer and the like, if necessary, are laminated. Laser light is irradiated on the surface of this recording layer to form pits on the portions of the irradiated surface to record information with a high density.
Since organic dye recording mediums represented by CD−R (write once CD: Compact Disc, registered trademark) are simple in arrangement and can be manufactured inexpensively, they are widely used on the market as mediums for backing up data, image data and music data. However, in accordance with rapid development of computers in recent years, it is desired that the storage capacity of recording media should be increased more. Therefore, a demand of the optical recording medium using the organic dye is being shifted from the CD−R to DVD−R and DVD+R (write once DVD: Digital Versatile Disc, registered trademark).
Information is recorded on the optical recording medium using the organic dye in the heat mode. When laser light is irradiated on the recording layer, the organic dye on the irradiated portion absorbs the laser light and it is heated and thereby decomposed. The portion in which the organic dye is decomposed is different from the portion which is not decomposed in reflectivity so that pits are formed in the recording layer. For this reason, a heat absorption mechanism becomes important in the optical recording medium using the organic dye. A heat quantity necessary for decomposing the organic dye in the recording layer depends on absorption spectrum, whereby laser irradiation power is adjusted.
In the optical recording medium using this organic dye, a reflectivity characteristic and a recording sensitivity characteristic of the recording layer are important. These characteristics are deeply correlated with each other, which may exert a large influence on the recording characteristic of the optical recording medium. The DVD uses laser light with a wavelength of 650 nm (DVD−R) or 655 nm (DVD+R) which is shorter than that of the CD in order to increase the recording density. The organic dye that has been used so far in the CD−R has a large absorption in this wavelength region and its recording sensitivity is considerably shifted from the optimum recording sensitivity so that it cannot be used in the DVD−R and DVD+R.
Further, the DVD−R and DVD+R also are requested to perform high-speed recording similarly to the CD−R. The recording material for carrying out high-speed recording is requested to have a high reflectivity, a high recording sensitivity and a high decomposition speed for decomposing the dye by heat.
Various kinds of dye materials suitable for use with the DVD−R and the like have been proposed (see Cited Patent References 1 and 2, for example).
However, it cannot be said that recording and reproducing characteristics in the high-speed recording mode have been examined enough.
Further, if the recording mode is optimized as the high-speed recording mode by simply increasing the decomposition speed of the dye, then the signal characteristic in the low-speed recording mode is deteriorated unavoidably. Otherwise, if the recording mode is optimized to the low-speed recording mode, the signal characteristic in the high-speed recording mode is deteriorated. Accordingly, it is very difficult to maintain the recording characteristic from the low-speed recording mode to the high-speed recording mode by a single organic dye recording medium.
[Cited patent reference 1]: Official gazette of Japanese laid-open patent application No. 2002-52829
[Cited patent reference 2]: Official gazette of Japanese laid-open patent application No. 2003-231359